This invention relates generally to apparatus for securing bone and ligament grafts to a bone mass and more particularly to an orthopedic fixation screw for holding the graft inside a bore.
The anterior cruciate ligament (ACL) is frequently injured in contact sports and other activities. Such injuries cause instability in the knee to an extent that ACL reconstruction may be required.
In ACL reconstruction, a substitute ligament or graft is attached to the distal femur or proximal tibia to facilitate regrowth and permanent attachment. Various methods of graft attachment are known, such as staples and sutures, however, such attachment methods are often not strong enough to withstand normal stress placed on the ligament.
One method for increasing the strength of the graft attachment comprises wedging an interference screw between a graft bone block and an interior wall of a bore formed though the bone mass. Although interference screws are stronger than other ligament attachment methods, such as staples, the sharp outer edges of the screw often cut or fray the ligament after the screw is fixated inside the bore.
U.S. Pat. No. 5,211,647 to Schmieding describes a sheath used for protecting a ligament graft during the insertion process of an interference screw. The sheath contains a cutout portion that enables the interference screw to be exposed to the tunnel wall of the bone during insertion, while at the same time covering the side of the screw facing the graft. The interference screw contains a rounded back section to further reduce the effects of graft cutting.
Because there are no threads at the back end, the screw shown in Schmieding has reduced purchase over fully threaded screws of comparable size. Reduced purchase in the screw increases the chance that the ligament graft may slip in the bore. Further, the screw in Schmieding has a cylindrical root portion that provides only nominal compression of the graft against the inside wall of the bore.
A constant root diameter also exhibits substantially constant resistance when inserted by a surgeon into the bore. Thus, it is hard for the surgeon to physically gauge exactly how far the interference screw has been inserted into the bore. For example, it is important that the screw is not inserted too far past the back end of the bore, while at the same time important that the screw does not extend partially out of the bore after completion of the installation process. The screw in Schmieding, however, gives no physical indication that insertion is near completion.
Accordingly, a need remains for a high purchase interference screw that will not cut or fray an attached ligament graft.